The invention relates to a method and a device for measuring/adjusting a voltage of an internal reference voltage source of an integrated semiconductor circuit, in particular, of a dynamic semiconductor memory. The reference voltage to be measured is compared by a comparator with a comparison voltage supplied from the outside, and the measurement result is formed in accordance with the result of the comparison.
German Patent DE 199 60 244.1 describes a configuration for trimming reference voltages in semiconductor chips.
In particular, in the case of integrated memory modules, for example, dynamic semiconductor memories (DRAMs) it is necessary to adjust or trim the setpoint voltage of voltage generators located on the chip, which fluctuates due to fabrication tolerances. According to the current state of the art in circuit technology, it is not possible to make available such voltage on the chip with greater precision than xc2x110% without an adjusting or trimming procedure.
However, before such a trimming or adjusting procedure can be carried out, the chip-internal reference voltage must be measured.
In order to improve the measuring accuracy, the reference voltage is currently measured using an external test system. The voltage can be varied in a specific range using software registers. The suitable values in the registers are determined from the measured value in the test system to arrive at the correct internal voltage, and the suitable values can be burnt into the chip permanently by laser fuses.
Such a method contributes considerably to the test costs: during the measurement of the internal reference voltage of SDRAM modules, sixty-four (64) modules, for example, are measured in parallel per wafer. Thus, it is necessary to determine sixty-four (64) analogous voltages with a high degree of accuracy by the test system. In the case of embedded DRAMs, there are a plurality of reference voltages on one module, for example, eight (8) reference voltages. Experience has shown that considerable fluctuations in reference voltages also occur within the module. In such a case, eight (8) analogous reference voltages must be measured per module.
With the concept proposed in German Patent DE 199 60 244.1 specified above, it is possible to standardize an internal voltage by making an internal digital/analog converter run through various values of a correction voltage.
In addition, it is possible to measure an internal voltage by searching for an external voltage that corresponds to the internal reference voltage. For such a method, just one binary output has to be led out from the chip. To compare the external voltage supplied from the outside with the internal reference voltage, an operational amplifier serving as a comparator must have a very high degree of accuracy because the offset voltage is completely absorbed in the set value for the reference voltage.
However, in customary operational amplifiers (in particular, CMOS operational amplifiers), it is possible, without a large degree of expenditure on circuitry, for considerable offset voltages to occur that are known to be the consequence of parameter variation in the transistors of the amplifier. If it is desired to minimize such parameter variations, the operational amplifier becomes very complex and costly.
It is accordingly an object of the invention to provide a method and device for offset-voltage-free voltage measurement and adjustment of a reference voltage source of an integrated semiconductor circuit that overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and that avoids the described problem of the offset voltage of the detecting operational amplifier so that a cost-effective operational amplifier can be used.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method for measuring and setting of a voltage of an adjustable internal reference voltage source of an integrated semiconductor circuit including the steps of comparing a reference voltage to an external comparison voltage with a comparator having two inputs, forming a measured value for the reference voltage that is to be set in accordance with a result of the comparison, switching a commutator by one of a clock-control and a software-control to alternatively apply the reference voltage and the external comparison voltage to the two inputs of the comparator at the same time, varying one of the reference voltage and the external comparison voltage in a direction of a setpoint voltage value until the comparator output changes its logic value at each switched stage of the commutator, buffering the voltage values present for each switched state of the commutator when the logic value of the comparator output changes and respectively varied in the preceding step, forming an average value for the reference voltage from the stored voltage values, and setting the reference voltage as a function of the average value formed in the preceding step. Preferably, the integrated semiconductor circuit is a dynamic semiconductor memory.
By the method according to the invention it is possible for the voltage value to be measured independently of the offset voltage of the operational amplifier serving as the comparator, by virtue of the fact that a change-over switch or commutator is provided upstream of the operational amplifier that functions as a comparator and the commutator alternately applies the reference voltage to be measured and the comparison voltage supplied from the outside to the comparator inputs. The measured value determined for the respective switched position of the commutator is stored in a piece of software that runs in a control unit forming, for example, part of a test equipment unit, and an average value is formed from the measured value. Thus, by software, the problem of the offset voltage of the comparator is avoided.
The method according to the invention proposes that the offset problems be avoided by a software solution. As such, the offset voltage is carried out by an offset elimination effectuated by software.
In accordance with another mode of the invention, it is also possible and, in view of the unavoidable RC characteristics of an external comparison voltage, possibly advantageous to vary the external comparison voltage incrementally and to place the commutator or change-over switch upstream of the comparator in both settings for each value of the comparison voltage.
In accordance with a further mode of the invention, it is also possible to fix the external comparison voltage at a setpoint value for the internal voltage, and to vary the internal reference voltage.
In accordance with an added mode of the invention, the varying step is carried out by maintaining the internal reference voltage constant while varying the external comparison voltage.
In accordance with an additional mode of the invention, the switching and varying steps are performed by, in a first switched state of the commutator, firstly varying the external comparison voltage in a voltage range around the setpoint voltage value with the reference voltage kept constant, and performing the buffering step by buffering that voltage value of the external comparison voltage at which the comparator output changes its logic state, and, in the second switched state of the commutator in which the reference voltage and the external comparison voltage are interchanged between the two comparator inputs in comparison with the first switched state of the commutator, varying the external comparison voltage in a voltage range around the setpoint voltage value with the reference voltage kept constant, and performing the buffering step by storing that voltage value of the external comparison voltage at which the comparator output changes its logic state.
In accordance with yet another mode of the invention, the external comparison voltage is incrementally varied.
In accordance with yet a further mode of the invention, the two switched states and of the commutator are assumed for each voltage value of the external comparison voltage.
In accordance with yet an added mode of the invention, the external comparison voltage is kept constant at a setpoint value for the internal reference value while the internal reference voltage is varied.
In accordance with yet an additional mode of the invention, the switching and varying steps are performed by, in a first switched state of the commutator, varying the reference voltage in a voltage range around the voltage value of the external comparison voltage that is kept constant with the external comparison voltage kept constant, and performing the buffering step by storing that voltage value of the reference voltage at which the comparator output changes its logic state, and, in the second switched state of the commutator, varying the reference voltage in a voltage range around the voltage value of the external reference voltage that is kept constant with the external comparison voltage kept constant, and performing the buffering step by storing that voltage value of the internal reference voltage at which the comparator output changes its logic state.
In accordance with again another feature of the invention, the internal reference voltage is incrementally varied.
With the objects of the invention in view, there is also provided a device for carrying out the method for measuring and setting of a voltage of an adjustable internal reference voltage source of an integrated semiconductor circuit including an internal reference voltage source providing an internal reference voltage, an external comparison voltage source providing an external comparison voltage, a comparator having two comparator inputs, a commutator having two commutator inputs and two commutator outputs, each of the two commutator inputs connected to a respective one of the internal reference voltage source and the external comparison voltage source, and each of the two commutator outputs directly connected to the two comparator inputs, the commutator alternately switching the two comparator inputs between the internal reference voltage and the external comparison voltage.
In accordance with again a further feature of the invention, there is provided a control unit connected to the commutator for switching the commutator
In accordance with again an added feature of the invention, the program proposed for software-supported offset elimination resides preferably in a control unit that also switches the commutator into the two settings.
It is advantageous that only a small portion of the components necessary for the voltage measurement have to be located on the chip. As such, in accordance with again an additional feature of the invention, at least the commutator and the comparator are provided on the chip of the integrated memory module. As mentioned, the comparator can be implemented by a cost-effective operational amplifier that takes up little chip area.
In accordance with still another feature of the invention, the control unit can be implemented as a program-controlled processor unit programmed to store the voltage values present for each switched state of the commutator and to form an average value for the reference voltage from the stored voltage values.
Preferably, in accordance with still further features of the invention, the processor unit has a storage device or means for storing the voltage values, and/or an average value forming device or means for forming an average value for the reference voltage from the stored voltage values.
In accordance with still an added feature of the invention, the control unit is preferably part of an external test equipment unit or measuring equipment unit for testing the integrated semiconductor circuit.
With the objects of the invention in view, there is also provided a device for measuring and setting of a voltage of an adjustable internal reference voltage source of an integrated semiconductor circuit, in particular, a dynamic semiconductor memory, including an internal reference voltage source providing an internal reference voltage, an external comparison voltage source providing an external comparison voltage, a comparator having two comparator inputs and a comparator output for outputting a logic value, a commutator having switched stages, a switch input, two commutator inputs, and two commutator outputs, each of the two commutator inputs connected to a respective one of the internal reference voltage source and the external comparison voltage source, each of the two commutator outputs directly connected to a respective one of the two comparator inputs, the commutator alternately switching the two comparator inputs between the internal reference voltage and the external comparison voltage, the comparator comparing the internal reference voltage to the external comparison voltage, a switch control having a switch output connected to the switch input of the commutator for alternatively applying the internal reference voltage and the external comparison voltage to the two comparator inputs at the same time, and one of the internal reference voltage and the external comparison voltage are varied in a direction of a setpoint voltage value until the comparator output changes the logic value at each switched stage of the commutator.
In accordance with still an additional feature of the invention, a control unit is configured to form a measured value for the internal reference voltage that is to be set in accordance with a result of a comparison of the comparator, to buffer voltage values present for each of the switched states of the commutator when the logic value changes and the one of the internal reference voltage and the external comparison voltage is varied, to form an average value for the internal reference voltage from stored voltage values, and to set the internal reference voltage as a function of the average value formed.
In accordance with a concomitant feature of the invention, the switch control is a clock-control or a software-control
With the features described above, the invention has in particular the following advantages:
The internal reference voltage can be measured independently of the offset voltage of the detecting operational amplifier and, subsequently, given a permanent setting. The error that occurs here in the voltage measurement can easily be kept within the submillivolt range.
The operational amplifier used is cheap and takes up only a small chip area.
The self-alignment logic can be tested because the data polarity has to be different depending on the position of the polarity switch.
As a result of the offset-tolerant method of voltage measurement, a high common mode range of the operational amplifier that serves as a comparator can be utilized.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and device for offset-voltage-free voltage measurement and adjustment of a reference voltage source of an integrated semiconductor circuit, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.